1. Field of the Invention
The present invention generally involves the field of technology pertaining to devices for separating and classifying particulate materials. More specifically, the invention relates to such a device in the form of a screen made of synthetic material and a method for making same.
2. Description of the Prior Art
The field of prior art pertaining to screens and similar appliances for separating and classifying particulate materials according to their relative sizes has been quite active and is well developed.
A specific application of such screens is in the mining and quarrying fields wherein particles of rather hard substances, such as coal, ores and rock, must undergo separation and classification for various purposes. Screens used for this purpose are necessarily subjected to severe wear conditions due to the abrasive nature of the hard particulates being processed. It is known to make these screens from either metal or synthetic materials. Metal screens are made by simply drilling or punching a metal plate to provide the necessary perforations. Synthetic screens are made by molding plastic material in a press under heat and pressure, with the perforations being formed either during the molding process or through subsequent drilling or punching of the molded body. An example of this technique wherein the perforations are formed during the molding process is disclosed by the Kramer U.S. Pat. No. 3,843,476.
The aforedescribed techniques for producing screens are characterized by fundamental disadvantages. The mechanical drilling or punching of perforations in a screen body is both time consuming and expensive. The simultaneous molding of perforations during molding of the screen body is less labor intensive and fairly economical, but the resulting perforations are not uniform in configuration and dimensions since the synthetic material is inevitably displaced in the regions of the perforations, particularly when a composite screen body formed from plural layers of different synthetic materials is being molded.
In addition to the above considerations, another important factor having a bearing on the performance of a screen is the specific disposition and configuration of the perforations. It is known to provide a screen with spaced perforations disposed in either linear arrays or staggered rows, and wherein the perforations may be any of a variety of different transverse cross-sectional configurations, including rectangular, round, hexagonal and variations of same. It has further been recognized as advantageous to provide each perforation with a substantially conical-shaped longitudinal cross-sectional configuration for defining relief angles to minimize the possibility of blinding the perforations since particulate material is always passing through a progressively larger opening. However, when a screen is disposed in a conventional inclined position of use, the particles being processed travel downwardly along the incline and impact against the top surface of the screen. Depending upon the angle of impact, the top openings of the perforations shall appear foreshortened in varying degrees to the particles, and consequently become reduced in effective screening size. Moreover, the relief angles defined by those wall portions of the perforations disposed downstream of the inclined screen are subjected to an accelerated rate of wear which exceeds that realized by their opposed upstream counterparts.